Propagation delay and skew

Last week, we considered the issue of propagation delay - the time it takes for a signal to travel along a data cable. We determined that differences between copper and fiber cable were not significant enough to have an impact on network performance. This week, however, we'll consider another issue of propagation delay that can be very significant.

It's called "skew" and it refers to the difference in propagation delay between different twisted pairs in a copper cable. For instance, in a four-pair Category 5 cable, skew defines the differences in transit time along each of the four pairs. Ideally, signals that enter the cable at the same time should arrive at the other end of the cable at same time. However, variations in manufacturing and cable composition can alter the propagation delay of different pairs.

As an example of how skew can affect performance, some high-quality Category 5 cables have four pairs coated with DuPont Teflon. This results in nearly identical propagation delay along all of the four pairs; that translates to very low skew.

In other cables, to save costs, only one or two pairs are coated with Teflon. Since Teflon-coated pairs generally have a lower propagation delay, signals on the Teflon pairs arrive slightly sooner than signals on the other pairs. Depending upon a number of factors, the skew can occasionally be quite high. How high? In some first-to-market cables with mixed construction, the skew was high enough to fail Category 5 specifications and even cause some high speed connections to fail.

Fortunately, today's Category 5 cables, even those with mixed Teflon and non-Teflon pairs, are designed to account for skew and meet Category 5 specifications. However, customers with older cables labeled "Category 5" would do well to check cable skew before running high-speed topologies across those cables.

Unfortunately, many cable testers don't measure skew. What can you do? In our next issue, we'll look at a simple "rule of thumb" that can infer skew from other measurements that cable testers do provide.

The Tolly Group, a strategic consulting and independent testing organization, offers a full range of services designed to furnish the vendor and the end-user communities with authoritative and unbiased information. The Tolly Group is a leader in assessing leading edge technologies and provides such services as: network design, product evaluations, industry studies, and market research. For more information, visit The Tolly Group's World Wide Web site, send e-mail to info@tolly.com, call 800-933-1699 or 732-528-3300, or fax 732-528-1888.